On Wednesday on an urban block in Baltimore, local TV news crews rushed to the site of a disaster: Several cars and half the street had tumbled into a railway ravine. No one was injured in what they took to calling the "Baltimore Sinkhole."

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In fact, this was no sinkhole. Just your standard landslide — one that residents of that street had seen coming for a while. "My wife and I haven't been parking on that side of the street for years because we knew it was going to happen," one local resident told The Baltimore Sun. So how did it happen, and why all of a sudden on Wednesday?

A landslide is a more straightforward geological phenomenon than a sinkhole. The key ingredients are soil, a hillside, gravity, and a catalyst to get the whole thing moving. Earthquakes and explosives used in mining can do it by rattling the earth loose; seismic activity is among the most common causes. Chopping down trees can do it, too, by killing roots that help anchor the soil in place. In this case, initial reports have suggested that the cause was a lot of rain, and a weak retaining wall that was supposed to keep the hill from rolling onto the CSX tracks below. When the ground becomes saturated with water, it becomes heavier and gravity does the rest.

The mayor of Baltimore said city engineers were reviewing maintenance reports to find out if it could have been prevented. Residents said they'd seen a crack emerge in the pavement long before the hill gave way. No one was injured in this landslide, unlike the recent one in Washington that killed 29 people. In that case, some had speculated that logging or an earthquake caused it. But the U.S. Geological Survey concluded the cause there was also heavy rain.

A sinkhole is a different story. There was the infamous case a year ago in Florida in which a man was sleeping in his Tampa-area home when the Earth opened up beneath his bedroom. He died in a 30-feet-deep by 30-feet-wide hole in the ground. Florida is especially prone to sinkholes because of the limestone that underlies most of the state. But there are other types of rock that can lead to sinkholes, which can happen anywhere from Montana to Texas to New York. When acidic groundwater eats away at layers of rock beneath the soil, the rock can collapse like thin ice, pulling down everything above it.

Dozens of people have been posting pictures of the Baltimore landslide on Twitter, as CSX races to clean up the damage.